This invention relates to superimposition of television images, and more particularly to the field of using video tape recording to superimpose images on an original master recording.
Visual displays are frequently used to present images of patterns or scenes lying outside the range of normal human visual perception. In most cases the images vary temporally while presenting a comparatively high density of information in the form of partially disguised objects of interest, of which only a select few are actually being sought. Human factors analysis attempts to quantify the ability of the human observers to detect and distinguish important aspects of objects dispursed within such images, in a form readily amenable to comparison.
In the course of such analysis it is necessary to record, reproduce and selectively alter images while retaining a high degree of temporal precision in the composite signals. Included in these modifications of the master image are cueing techniques, to supplement the human sense with the high speed electronic analysis of video signals, slow-motion reproductions, split screen presentations, and built-in self test modes. The overriding consideration in all the approaches is the need for synchronization with, and repeatability of, the master video signal.
Insertion of cueing signals into or upon a master image for purposes of designation provides an example through which deficiencies in conventional recording and reproduction systems can be distinctly displayed. First, consider the use of a video magnetic disc type recorder. Though a copy of the master video, with cueing signals inserted, could be generated for recording onto such a recorder, the present technology limits operating time to approximately 30 seconds. Similarly, if cueing or other signal modifications are to be extracted from such a magnetic disc recorder for use during display, the severe time restriction also exists. U.S. Pat. No. 4,157,572 is a case in point, where a video magnetic disc recorder is utilized, but by nature of its operating time restriction provides no more than a cyclic repeat of a single video presentation.
The helical scan represents the remaining technological standard in video recorders. Though it overcomes the operating time restrictions of the video disc type, the helical scan recorder is inherently restricted in other ways from direct use in the application described. As a first instance, helical scan recorders are, as a general rule, limited to one signal channel. Temporally concurrent signals which are spatially synchronized to the master video are not feasible with the same tape, while using multiple video tapes and recorders will obviously succumb to synchronization problems. Though a multiple channel helical scan video recorder may overcome synchronization degradation, slow motion effects are limited to those which can be generated by undertaking multiple, successive scans of identical frames or fields. As such, the flexibility inherent in using an X-Y monitor for slow motion reproduction, in place of the conventional raster scan, is not attainable.
Undoubtedly, one moderately skilled in the art recognizes that the well known longitudinal recorder has not been discussed for video recording and reproduction. The lack of extensive use tends to focus on two well known deficiencies, which are in part interrelated. First, recording of the composite video signal in conventional manner creates jitter during reproduction, a phenomenon attributable to deficient low frequency response in comparison to the precision required to maintain accurate vertical synchronization. Stated otherwise, the broad frequency bandwidth of the composite video signal is fundamentally incompatible with the limited frequency capabilities of the longitudinal recorder, appearing in most cases as a low quality reproduction of the lower end frequencies. The second, and equally distasteful aspect of using a longitudinal recorder for composite video signals, lies in the high tape speed rate necessary to record and reproduce video signals with reasonable accuracy. Undoubtedly, tape speed can be reduced at the sacrifice of picture quality. However, the above noted problem with vertical synchronization, as well as horizontal synchronization, rapidly bound the reduction that can be achieved. Even nominal errors in synchronization destroy the whole video presentation.